1. Field of the Invention
The present invention generally relates to the field of switches, and more specifically to the making of switches adapted to switching a circuit connected to the mains in a way compatible with present standards relative to electromagnetic disturbances.
2. Discussion of the Related Art
To switch a high mains voltage, thyristor-type switches which are normally off (which have to be controlled to be turned on) or assemblies which are normally on (which have to be controlled to be turned off) are currently used. The turn-on or turn-off control of these switches creates a strong current variation during a very short time interval. This abrupt variation is likely to cause electromagnetic disturbances in neighboring equipment.
FIG. 1 illustrates an example of normally-on switch assembly. It is desired herein to control the supply of a load 1 intended for receiving part of the power that an A.C. supply voltage source Vac, for example the mains voltage, applied between two terminals 2, 3, can provide. The power provided to load 1 is controlled by a switching circuit 4. Circuit 4 includes a switch 5 and a control circuit 6. Switch 5 is a gate turn-off thyristor (GTO), provided with anode and cathode gates which are interconnected by a resistor R1.
FIG. 2 shows the shape of current I in the load as a function of time. Due to the presence of resistor R1, the GTO thyristor naturally turns on at the beginning of each positive halfwave of voltage Vac (times t1). Circuit 6 is meant to turn off the GTO thyristor when supply voltage Vac reaches a predetermined level. In the illustrated example, circuit 6 includes a reference diode or avalanche diode Z, the cathode of which is connected to the midpoint of a resistor bridge R2, R3. The values of resistors R2, R3 are chosen so that diode Z starts an avalanche when voltage Vac reaches the level at which the supply of load 1 is desired to be interrupted. Between the cathode gate of the GTO thyristor and terminal 3 is connected a switch 7, for example a thyristor, which turns on when diode Z becomes conductive, after which switch 5 turns off (time t2) and the current through the load abruptly drops to zero, as illustrated in FIG. 2 in dotted lines. Then, the current remains at zero until the beginning of the next positive halfwave where this sequence is repeated.
As mentioned previously, the abrupt variation of the current on the supply line at times t2 can cause electromagnetic disturbances.
To solve this problem, a low-pass filter 8 is conventional provided between each of terminals 2, 3, and circuit 4. Filter 8 includes, for example, inductors L1 and L2 and a capacitor C. A first end of each of inductors L1, L2 is connected to one of the respective terminals 2, 3. The second ends of inductors L1 and L2 are interconnected by capacitor C and are respectively connected to the terminals of the series circuit including the GTO thyristor and load 1. For a 1-A switched current, the values of inductors L1, L2 typically are on the order of 150 .mu.H, and the capacitance of capacitor C typically is 470 nF. A disadvantage of this type of conventional solution is that such a filter 8 is bulky, non-integrable and expensive.